In conventional in situ leaching processes a lixiviant, also called a leach solution, is pumped into a mineral-bearing formation through an injection well, and a pregnant lixiviant, or leachate, containing the mineral value is removed from the formation through one or more production wells. This method is especially valuable in the recovery of uranium, as discussed in R. W. Merritt, The Extractive Metallurgy of Uranium 108-112 (1971).
When in situ leaching is practiced in the field, it is observed that the leachability of uranium ores varies significantly among ore bodies, regardless of the lixiviant system used. The reasons for these variations are not known. The complexity of the problem is aggravated by the wide variety of ore body mineralogy and geology, as well as by the variety of leaching systems currently available. Some of the solutions proposed involve the use of more powerful oxidants or heat to increase leaching rates, but these approaches tend not only to increase process costs but also to introduce new environmental and process difficulties that familiar leaching systems such as CO.sub.2 /O.sub.2 do not pose.
In typical practice, the pumping rate, and thus the residence time of the lixiviant in the formation, is maintained at a constant level throughout the entire period of the leaching operation. As a result, the concentration of uranium in the leachate decreases continuously as the uranium in the ore body is depleted, causing a reduction in production of yellowcake and increased costs per pound of useable uranium.